The objective of the proposed work is to investigate kinetics of proliferation and differentiation of the Type 2 alveolar cell, preceeding development of Type 2 cell adenomas induced by urethane. Morphometric and autoradiographic techniques will be used to determine cell cycle time, population growth, life span of Type 1 and Type 2 cells, and division-differentiation of Type 2 stem cell both in normal mouse lung and in lung during preneoplastic hyperplasia initiated by urethane.